Land chromium-side work to clean up handling of v8_target_cpu in the GN build.

build/config/v8_target_cpu.gni

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
-
-import("//build/config/sanitizers/sanitizers.gni")
-
-declare_args() {
-  # This arg is used when we want to tell the JIT-generating v8 code
-  # that we want to have it generate for an architecture that is different
-  # than the architecture that v8 will actually run on; we then run the
-  # code under an emulator. For example, we might run v8 on x86, but
-  # generate arm code and run that under emulation.
-  #
-  # This arg is defined here rather than in the v8 project because we want
-  # some of the common architecture-specific args (like arm_float_abi or
-  # mips_arch_variant) to be set to their defaults either if the current_cpu
-  # applies *or* if the v8_target_cpu applies.
-  #
-  # TODO(crbug.com/620527) - rework this whole approach so that it isn't
-  # v8-specific.
-  v8_target_cpu = ""
-}
-
-if (v8_target_cpu == "") {
-  if (is_msan) {
-    # Running the V8-generated code on an ARM simulator is a powerful hack that
-    # allows the tool to see the memory accesses from JITted code. Without this
-    # flag, JS code causes false positive reports from MSan.
-    v8_target_cpu = "arm64"
-  } else {
-    v8_target_cpu = target_cpu
-  }
-}
+#
+# TODO(crbug.com/621581): Remove this file once v8 no longer references it.